Electrical signaling system



Feb. 4, 1941. A c CORNER 2,230,937

ELECTRICAL SIGNALING SYSTEM Filed Dec. 2, 1939 LPF/ L PR3 LPFZ 010m HPF/ 0pm 'HPF3 IVD/IZ LID/'4 W 3000 T 5/00 4900 4900 3000 w INVENTOR ARTHUR CHARLES CORNER Patented Feb. 4, 1941 UNITED STATES PATENT OFFICE V I 2,230,937 ELECTRICAL SIGNALING SYSTEM British company Application December 2, 1939, Serial No. 307,217 In Great Britain November 5, 1938 8 Claims.

The present invention relates to electrical signaling systems and is more particularly concerned with systems employing carrier currents for transmitting speech and signal currents between a plurality of stations.

It has previously been proposed to provide a single source of carrier current for common use by two stations and in order to ensure that adequate power is supplied to the stations the source of current has been applied at the electrical centre of the line. Since it is not always convenient to apply the source 'at the geographical centre of the line, which normally is also the electrical centre, the source has been applied to the line at one of the stations and a balancing network has been inserted in the line betweenthe source and the local modulator so that the point of application of the source is at the effective electrical centre of the line.

The addition of such a network however has the disadvantage that it causes an increase in the overall attenuation of the circuit and it is an object of the present invention to provide an arrangement whereby the source of carrier current is located at one station and adequate power is supplied to the distant station without the use of such a balancing network.

According to this feature of the invention the carrier oscillations for the intercommunicating stations are at opposite sides of a network so arranged as to prevent the flow of carrier current and to permit the flow of one sideband current, the level of carrier oscillations for each station being determined independently for instance by separate amplifiers.

The network between the two points takes the form of a high pass filter when the upper sideband is to be transmitted and a low pass filter when the lower sideband is to be transmitted. The cut-off frequency of the high pass filter will be just above the carrier frequency and that of the low pass filter just below the carrier frequency.

According to a further feature of the invention it is possible to employ both sidebands when the network could be a rejector circuitor preferably a combination low-pass and high-pass filter with a narrow stop range at the carrierfrequency. This employment of a single sideband has the advantage of eliminating the necessity of phase compensating networks which have hitherto been considered essential as when both sidebands are transmitted with the carrier it is necessary to maintain the correct phase relationship between them, otherwise the amplitude of the received speech will vary with the phase displacement whereas when a single sideband only is transmit-' v ted with the carrier the phase relationship does not matter since only the phase of the received speech is changed and this cannot be aurally detected.

A particular embodiment of the invention will be described with reference to the accompanying drawing in which the upper sideband is employed and in which the carrier and sideband are superimposed on a line used for speech currents.

HPFI represents the network which it is assumed to have a cut-off frequency of 5100 cycles per second at the opposite sides of which the carrier oscillations of 5000 cycles per second are applied from the oscillator OSC through the independently adjustable amplifiers Al and A2 whereby the level of the carrier oscillations can be set to that most suitable for the station which it feeds. It is assumed that the source of oscillations is provided at one of the stations although it could equally well be applied at an intermediate station if the two intercommunication stations had access to each other via the intermediate station.

. The telephones of the two stations are connected to the jack terminals shown in any appropriate manner and are separated from the modulator-demodulator units MDMI and MIDMZ by the low pass filters LPFZ and LPF I. The modulator-demodulator unitsas their name implies are adapted to function both as modulators and demodulators and are well known. Preferably they consist of dry plate rectifiers and they serve to modulate the outgoing speech currents and demodulate the incoming sideband currents. As shown the carrier current is supplied to the two units over the same pair of leads as the pair over which the sidebands are transmitted although where the source of oscillations is at the same station as the modulator-demodulatorunit it is possible for this unit to be of the six terminal type insteadof being of the four terminal type in which case the source of carrier oscillation can be connected up to' separate terminals as shown by dotted lines.

The low pass filters LPFI and LPFZ having a cut-off frequency of 3000 cycles per second serve to prevent the carrier currents and sidebands from affecting the speech frequency circuit while the high pass filters HPF2 and HPF3 having a cut-off frequency serve to ensure that approximately only currents of carrier frequency and above can reach the line from the carrier current units and can reach the carrier current units from the line.

A system of the type described above has particular application to rural automatic exchange systems in which the rural exchange is connected to the main exchange over a single junction line. Calls within the rural exchange are set up automatically while incoming and outgoing calls are set up by an operator at the main exchange over the junction line. Since the junction line may be of some considerable length the cost of installing more than one junction line may be prohibitive. With a system as described above an addition-al channel can be obtained over the existing junction line and since the carrier current source is located at the main exchange and dry plate rectifiers are employed at the rural exchange, the maintenance costs are small and no power supply beyond the usual exchange battery and ringing current source is required.

There is, however, a further problem which arises in connection with this particular application, namely the transmission of dialling and supervisory signals. of the carrier current is not suificiently high for satisfactory transmission of these signals over the carrier channel and a further object of the invention is the provision of separate channels for the transmission of dialling and supervisory signals.

According to a subsidiary feature of the invention dialing and supervision is effected over an alternating current loop circuit or over a phantom circuit or over the separate conductors of the line. In the case of the alternating current 100p circuit the signaling frequency may be below or within the voice frequency range and as regards the phantom circuit, alternating current below the voice frequency range may be employed or direct current with an earth return may be used, the application of the signaling currents to the line being effected over suitable relay sets to translate the loop signals, dialed impulses and supervisory and like signals into alternating current-signals for transmission to the other end of the line and there to translate them back into their original or another preferred form. Suitable filters are, of course, provided at each end to separate the various signaling currents.

For instance in one case dialing and supervision for the voice channel is effected by a sub-audio frequency transmitted over the loop circuit while for the carrier channel the same or a different sub-audio frequency is transmitted over the phantom circuit. In another alternative, dialling and supervision is effected by a voice frequency for the voice channel and a sub-audio frequency for the carrier channel both transmitted over the loop circuit. A direct current telegraph channel is then'obtained over the phantom. This arrangement has the disadvantage that a voice frequency generator has to be employed at the rural exchange. This, however, can be supplied by a battery driven vibrator since the actual frequency and wave form are unimportant.

In the case where signaling is effected over one conductor of the line for one channel and over the other conductor for the other channel the same or difierent sub-audio frequencies or direct current may be employed for each channel and a suitable low-pass filter serves to isolate the signaling circuits as regards speech. Further a high pass or a bandpass filter is employed in the voice channel to prevent the signal frequencies affecting the speech circuits.

It is found that the level,

It will be understood thatthe carriersystemcan be applied to circuits other than that described above. For instance the rural exchange may be an intermediate exchange and one of the channels may be extended to a further exchange. Further the rural automatic exchange may be located between two manual exchanges and communications between the automatic and the manual exchanges may be effected over voice frequency channels while the carrier channel may serve as'a direct communication channel between the manual exchanges or vice versa.

Finally the carrier system may provide for two independent carrier channels and in this case dialling and supervision for the two channels may be effected over a loop circuit and over a phantom circuit respectively.

I claim:

1. In combination in a carrier-wave signa system, a network designed to prevent the flow of carrier current while permitting the flow of sideband currents, leads extending from each side of said network for interconnecting two communicating stations, a source of carrier wave oscillations, separate paths to the leads on each side of said network from said source of carrier wave oscillations, modulator-demodulator units connected in the leads extending from each side of said network, one unit being located at each communicating station, said modulator-demodulator units in conjunction with the carrier wave oscillations received from said source at the respective stations being adapted to modulate the carrier wave oscillations with outgoing speech currents and to demodulate the incoming modulated waves to produce speech currents, and means in said paths whereby the strength of said carrier wave oscillations on the opposite sides of said network may be separately determined.

2. In combination in a carrier wave signaling system, a network designed to prevent the flow of carrier current while permitting the flow of one sideband current only, leads extending from each side of said network for interconnecting two communicating stations, a source of carrier wave oscillations, separate paths to the leads on each side of said network from said source of carrier wave oscillations, modulator-demodulator units connected in the leads extending from each side of said network, one unit being located at each communicating station, said modulator-demodulator units in conjunction with the carrier wave oscillations received from said source at the respective stations being adapted to modulate the carrier wave oscillations with outgoing speech currents and to demodula'te the incoming modulated Waves to produce speech currents, and means in said paths whereby the strength of said carrier wave oscillations on the opposite sides of said network may be separately determined.

' 3. In combination in a carrier wave signaling system, a network designed to prevent the flow of carrier current while permitting the flow of sideband currents, leads extending from each side of said network'for interconnecting two communicating stations, a source of carrier wave oscillations,'separate paths to the leads on each side of said network from said source of carrier wave oscillations, modulator-demodulator units con nected in the leads extending from each side'of said network,one unit being located at each communicating station, said modulator-demodulator units in conjunction with the carrier wave oscillations received from said source at the respective stations being adapted to modulate the carrier wave oscilla ions with outgoing speech currents and to demodulate the incoming modulated waves to produce speech currents, and an amplifier in each of said paths whereby the strength of said carrier wave oscillations on the opposite sides of said network may be separately determined.

4. In combination in a carrier wave signaling ssytern, a network designed to prevent the flow of carrier current while permitting the flow of sideband currents, leads extending from each side of said network for interconnecting two communicating stations, a source of carrier wave oscillations, modulator-demodulator units employing dry plate rectifiers connected in the leads extending from each side of said network, one unit being located at each communicating station, said demodulator units in conjunction with carrier wave oscillations received from said source at the respective stations being adapted to modulate the carrier wave oscillations with outgoing speech currents and to demodulate incoming modulated waves to produce speech currents, separate paths to the leads on each side of said network from said source of carrier wave oscillations, and means in said paths whereby the strength of said carrier wave oscillations on the opposite sides of said network may be separately determined.

5. A carrier wave signaling system comprising a line, a high pass filter connected to each end of said line to permit the passage of modulated carrier currents havin frequencies above speech frequency, a network designed to prevent the flow of carrier current while permitting the flow of sideband currents connected on one side to the side of one of said high pass filters opposite to that side which is connected to the line, a modulator-demodulator unit connected to the other side of said network and leads extending from said modulator-demodulator unit for connection to a communicating station, a source of carrier wave oscillations, separate paths to the leads on each side of said network from said source, means in said paths whereby strength of such carrier wave oscillations on the opposite sides of said network may be separately determined a second modulator-demodulator unit connected to the side of the other high pass filter opposite to that side which is connected to the side of the line other than that to which the first filter is connected and leads extending from said second modulator-demodulator unit for connection to a second communicating station.

6. A carrier wave signaling system comprising a line, a high pass filter connected to each end of said line to permit the passage of modulated carrier currents having frequencies above speech frequency, a network designed to prevent the flow of carrier current while permitting the fiow of one sideband current only connected on one side to the side of one of said high pass filters opposite to that which is connected to the line, a modulator-demodulator unit connected to the other side of said network and leads extending from to a communicating station,a source of carrier Wave oscillations, separate paths to the leads on each side of said network from said source, means in said paths whereby strength of such carrier wave oscillations on the opposite sides of said net-work may be separately determined, a second modulator-demodulator unit connected to the side of the other high pass filter opposite to that which is connected to the side of the line other than that to which the first filter is connected, and leads extending from said second modulatordemodulator unit for connection to a second communicating station.

'7. A carrier wave signaling system comprising a line, a high pass filter connected to each end of said line to permit the passage of modulated carrier currents having frequencies above speech frequency, a network designed to prevent the flow of carrier current while permitting the flow of sideband currents connected on one side to the a side of one of said high pass filters opposite to that side which is connected to the line, a modulator-demodulator unit connected to the other side of said network and leads extending from said modulator-demodulator unit for connection to a communicating station, a source of carrier wave oscillations, separate paths to the leads on each side of said network from said source, an amplifier in each of said paths whereby strength of such carrier wave oscillations on the opposite sides of said network may be separately determined, a second modulator-demodulator unit connected to the side of the other high pass filter opposite to that side which is connected to the side of the line other than that to which the first filter is connected, and leads extending from said second modulator-demodulator unit for connection to a second communicating station.

8. A carrier wave signaling system comprising a line, a high pass filter connected to each end of said line to permit the passage of modulated carrier currents having frequencies above speech frequency, a network designed to prevent the flow of carrier current while permitting the fiow of sideband currents connected on one side to the side of one of said high pass filters opposite to that side which is connected to the line, a modulator-demodulator unit employing dryplate rectifiers connected to the other side of said network and leads extending from said modulator-demodulator unit for connection to a communicating station, a source of carrier wave oscillations, separate paths to the leads on each side of said network from said source, means in said paths whereby strength of such carrier wave oscillations on the opposite sides of said network may be separately determined, and a second modulatordemodulator unit employing dry-plate rectifiers connected to the side of the other high pass filter opposite to that side which is connected to the side of the line other than that to which the first filter is connected and leads extending from said second modulator-demodulator unit for connection to a second communicating station.

ARTHUR CHARLES CORNER. 

